1. Field of the Invention
The invention relates to a method and a device for image data noise filtering, and to a display apparatus comprising such a device.
2. Description of the Related Art
In a motion adaptive first-order temporal recursive filter as proposed in References (1) and (2), for every pixel position x=(x, y).sup.T, with .sup.T indicating transposition, and an input luminance value F(x, t), the filter output F.sub.F (x, t) is defined as: EQU F.sub.F (x, t)=kF(x, t)+(1-k)F.sub.F (x, t-T) (1)
where k is a control parameter, defining the filter characteristics and T is the field period of the video signal, which equals 20 ms in a 50 Hz environment. In an interlaced scan environment'x has to be increased (x+(0, 1).sup.T) or decreased (x-(0, 1).sup.T) with one line, as the corresponding pixel in the previous field does not exist. In an advantageous implementation (Reference (3)), the vertical position is field alternatively increased or decreased: ##EQU1## where N.sub.F is the field number. The variable k is determined with a so-called motion detector, the processing of which can be expressed as: ##EQU2## where N.sup.1 and N.sub.2 are (usually small) neighborhoods around the current pixel, and LUT is a monotonous, non-linear Look-Up Table function that translates its argument into a value usually between 1/32 and 1.
Although the filter is adapted to perform less filtering in case of motion, see equation (3), usually some blurring of fine low-contrast detail is still visible. If the motion detector of equation (3) is set more sensitive in order to prevent this blurring, the noise reduction capability decreases dramatically as the noise itself is seen as motion.
Another disadvantage of the classical temporal filter described, is that it causes a "dirty window effect", i.e., the filter suppresses the higher temporal frequencies which "freezes" the noise on the screen. In undetailed moving areas, therefore, the noise appears as a dirt on the screen behind which the undetailed body moves.